U.S. Pat. No. 5,913,830 discloses an inductive plethysmography respiration sensor for measuring the change of circumference and cross-sectional area of the chest or abdomen. The sensor includes a conductor loop with (“inactive”) sections in which the conductor runs parallel to the direction in which the sensor expands and contracts when the person wearing it breathes and (“active”) sections in which the conductor comprises a sharp bend in the transversal direction. When the sensor is stretched, the bends change shape, leading to a change in inductance of the conductor loop.
Whereas U.S. Pat. No. 5,913,830 fails to disclose any details about the sensing circuitry, two types of circuits are conventionally used for measuring inductance changes. The first type of circuit is an LC oscillator that uses the sensing coil as tank element. The oscillator frequency is indicative of the inductance. This circuit has the disadvantage that the LC oscillator may lock its frequency to an external magnetic field with a frequency close to the tank frequency. This could happen, for example, when two persons wearing the same measurement device are close to each other, or when an electromagnetic field is present which is generated by a for example a radio transmitter operating at a frequency close to the resonance of the LC oscillator. The second type of circuit uses a voltage divider driven by a high-frequency periodic signal (e.g. at 1 MHz). The voltage divider comprises a reference impedance and the inductive element. The amplitude of the divider output voltage is indicative of the inductance. An inductive sensor has typically an inductance of about 1 μH and a variation of up to 100 nH when the subject is breathing. At a frequency of 1 MHz, an inductance of 100 nH corresponds to an impedance of 0.628 Ω. This implies that in order to extract a measurable signal, either a large current must be used to generate a usable divider output voltage, conflicting with the low power requirement, or a very small output voltage must be amplified, conflicting with the low power and low cost requirements, or a higher operating frequency must be employed, which conflicts with the low power requirement.